A weird window decoration problem occurs with Firefox when launched in a maximized state with Title bar disabled. It opens fine, but once resized it has this odd extra decoration on the window (see the picture). The bug doesn't go away when maximized again and occurs for all new windows launched in a maximized state.

When Firefox operates with Title bar enabled, it behaves as expected.

I have Global Menu and the WIndow Picker applet in the top panel. The Global Menu applet crashes often when I switch applications quickly. The window decoration problem occurs on Marco with or without Compton, as well as on Compiz.

The weirdest thing is that on my main system it doesn’t happen. If I clone my whole home directory to a new user, it still won’t happen, but I haven’t been able to track it down to one setting or config file. It’s not just my Firefox profile.

Thanks. I think for now I will just keep Firefox with a Title bar and then disable window decoration for maximized windows in MATE Tweak. The WIndow Picker applet and keyboard shortcuts take care of closing, minimizing and restoring maximized windows then.